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Members of the INER Working Group on Influenza are listed in the Appendix.
In April 2009, the Mexican Secretariat of Health reported an outbreak of respiratory disease. In the affected patients, a novel swine-origin influenza A (H1N1) virus (S-OIV) with molecular features of North American and Eurasian swine, avian, and human influenza viruses1 -4 was found. In the same month, the World Health Organization (WHO) classified the global spread of this virus as a public health event of international concern. After documentation of human-to-human transmission of the virus in at least three countries of two WHO regions, the WHO raised the pandemic level to 6.5
As of May 29, 2009, Mexico had reported 4910 confirmed cases and 85 deaths caused by S-OIV.6 Mexico has reported the greatest number of cases of severe clinical presentations and death,1 whereas other countries have reported predominantly mild cases of influenza-like illness.
This case series describes the clinical and epidemiologic characteristics of the first 18 persons with pneumonia and laboratory-confirmed S-OIV infection (also known as swine flu) hospitalized at the National Institute of Respiratory Diseases (INER) in Mexico. We also describe apparent transmission of this infection to health care workers during the initial days of the outbreak.
Methods
INER is the Mexican national tertiary care and research center devoted to respiratory diseases. The 178-bed facility provides clinical services primarily for the uninsured population of Mexico City and neighboring states. We retrospectively reviewed medical charts and radiologic and laboratory findings. This study was determined to be exempt from the requirement of institutional review, because it was conducted as part of a public health investigation into retrospective data. All tests and procedures were performed at the request of the physicians in charge of the patients. All study patients had influenza-like illness with opacities found on the chest radiograph (revealing pneumonia) and had laboratory-confirmed S-OIV infection. We also reviewed clinical data from a group of 21 hospitalized patients with influenza-like illness and pneumonia but with a negative result on reverse-transcriptase-polymerase-chain-reaction (RT-PCR) testing for influenza A (H1N1).
Microbiologic Studies
Nasopharyngeal-swab specimens were collected at admission, and bronchial-aspirate samples were obtained after tracheal intubation. Specimens were placed in transport medium and kept at a temperature from 2 to 4°C. RT-PCR testing was done in accordance...